Thursday, August 07, 2003

Looking at the Big Picture Through The Wrong End of the Telescope

This post at Bizarre Science (link via Troppo Armadillo) has prompted a little blogospheric warming over the past 24 hours. It refers to this article recently published in on the effects of supernovae on atmospheric temperature. This helpful digest of the article by Tim Patterson, author of the Bizarre Science post may be more accessible.

The story, in a nutshell is this. We all know that the Earth orbits the Sun. The Sun, in turn, orbits the centre of our galaxy the Milky Way. At least, this is the story according to the current high school science curriculum - give it another couple of hundred years, and the whole picture might have been brought up to date with General Relativity which, if you're optimistic about the progress of scientific knowledge will be as obsolete as the "little objects go round big objects" Newtonian view we're working with here. The pessimistic prospect that the whole of twentieth century astrophysics might equally well have been thrown out in favour of a theory in which the speed of light increases the further you get from the earth so that objects allegedly millions of light years away are only 6,000 light years away at most, doesn't bear thinking about. The mathematics of such a theory would make calculating a Lorenz-Fitzgerald contraction look easier than splitting the bill at a Sunday yum-cha.

So the Sun orbits the center of the galaxy, pulling the Earth and the other planets along with it. Its orbit takes it through regions of space which have a lot of stars in them (the galactic arms) and regions where there are fewer stars (the spaces between the arms). When the Earth is passing through one of the galactic arms, It gets more galactic cosmic radiation, which originates (apparently) from supernovae. A supernova is essentially the sort of big kaboom that would satisfy Marvin the Martian (remember Duck Dodgers in the twenty four and halfth Century?), although not the biggest of all possible kabooms. The biggest kaboom of all was probably the last, unless someone can come up with quite a few teratons of matter that's been otherwise unaccounted for because it's in some bizarre form that nobody has recognised yet.

At this point we can leave cosmology behind, and look at the effects of cosmic radiation from supernovae, and other (non Solar) sources, on the Earth's atmosphere. What it amounts to is this: cosmic radiation ionises aerosols (stuff suspended or "dissolved" in the atmosphere). Ionised aerosols attract water more readily than non-ionised aerosols, so clouds form more easily. Easier cloud formation means more cloud formation, and clouds relect light; in technical jargon, clouds increase the Earth's albedo, which means that if you have a lot of cloud cover over long periods, the atmosphere should, generally speaking, get cooler. So when the Earth is in one of the galactic arms, you get more exposure to galactic cosmic rays, which means more cloud cover which means lower global temperatures. And when the Earth is out in the galactic boondocks, you get less exposure to galactic cosmic rays, which means less cloud cover, which means higher global temperatures.

The authors of the article, Dr. Jan Veizer and Dr. Nir J. Shaviv, were able to show a 135 million year cycle of temperature increases and decreases. The evidence for the cycle came from analysing the fossilised remains of shellfish: lest this be taken as an invitation to Luddite skepticism its worth specifying that the analysis measured the concentration or relative quantity of Oxygen-18 in the fossilised shellfish. Let's assume, for the time being that the amount of oxygen with an atomic weight of 18 in a shellfish fossil reflects changes in global temperature. I doubt that reinforcing this by saying that I consider this perfectly plausible is going to work somehow: habitual irony has serious drawbacks when you're trying to make a serious statement for a change. Perhaps I should say that I don't consider it any less plausible than the accepted view that the speed of light is a physical constant that doesn't vary with distance from the Earth.

The case for galactic cosmic radiation as a driver of climate change, at least on geological timescales is largely made by statistical inference. Essentially, the authors are arguing that it's very likely that much of the climatic variation over the past 545 million years has been due to supernova activity, with only a 1.2% chance that the explanation is something else. In essence, arguing that the temperature cycle studied by Veizer and Shaviv was caused by something other than galactic cosmic radiation would be a little like backing a 100 to 1 outsider at Eagle Farm. That might be a sensible bet if the horse's name was Fine Cotton, but that's not the point of the analogy.

Does this finding shoot another big hole in the scientific basis for the Kyoto protocol? Veizer and Shaviv sound a couple of cautionary notes for anyone who wants to leap to this conclusion:

In summary, we find that with none of the CO2 reconstructions can the doubling effect of CO2 on low-latitude sea temperatures be larger than ~1.9 °C, with the expected value being closer to 0.5 °C. These results differ somewhat from the predictions of the general circulation models (GCMs) (IPCC, 2001), which typically imply a CO2 doubling effect of ~1.5–5.5 °C global warming, but they are consistent with alternative lower estimates of 0.6–1.6 °C (Lindzen, 1997).

As a qualifier, one should note that global temperature changes should exceed the tropical ones because the largest temperature variations are in the high-latitude regions for which we do not have any isotope record. A review of GCMs (IPCC, 2001) shows that the globally averaged warming from CO2 is expected to be typically 1.5 times larger than that of the tropical temperatures, and our model uncertainty limits should therefore be modified accordingly. [my emphasis]

As a final qualification, we emphasize that our conclusion about the dominance of the CRF over climate variability is valid only on multimillion year time scales. At shorter time scales, other climatic factors may play an important role, but note that many authors (see previous references) suggest a decisive role for the celestial driver also on multi-millennial to less than annual time scales. [my emphasis again]

Once again, it looks like the global warming debate hasn't been settled, despite the optimistic expectations of the global warming skeptics. At times like these, I like to remind myself that not only did the Montreal Accord finally get up, but according to the evidence that's now starting to emerge, the bloody thing worked.